Page 575 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 25 Tumors of the Skeletal System 553

the cartilaginous cap and the underlying stalk of bone. Histo- Periarticular Tumors
logically, this cartilaginous cap gives rise to an orderly array of Periarticular tumors in dogs include histiocytic sarcoma (HS),
synovial myxoma/myxoma sarcoma, malignant fibrous histiocy-
maturing bone according to the sequence of endochondral ossi-
VetBooks.ir fication. The cortical bone surfaces of the mass and the adjacent toma, and synovial cell sarcoma (SCS). Typical presenting clinical
353
bone are confluent.
A strong presumptive diagnosis is made
signs include lameness and a firm mass or swelling near the joint.
by evaluation of the history, physical examination findings, and These tumors typically occur in the large joints (particularly the
radiographic features. stifle) of the extremities and occur most commonly in large breed,
Treatment involves conservative surgical excision, but this is middle-aged dogs. Radiographs show lytic and/or proliferative
only necessary if clinical signs do not abate after the dog reaches changes in the bones surrounding the involved joint. The involve-
skeletal maturity. Because of the likelihood of a heritable etiology, ment of more than one bone near an affected joint distinguishes
affected dogs should be neutered. Owners should also be advised the radiographic appearance of a periarticular tumor from a pri-
of the possibility of late malignant transformation. Dogs with a mary bone tumor. The gold standard for the diagnosis of periartic-
previous history of MCE should be carefully evaluated for bone ular tumors is biopsy with IHC staining to differentiate the various
malignancy if clinical signs return later in life. tumor types. Classic IHC markers include cytokeratin, CD18,
vimentin, and smooth muscle actin. The distinction between SCS
Bone Cysts and periarticular HS is important as these two tumors have dif-
Cysts are rare, benign lesions of bone. The majority of the fering biologic behaviors and recommended treatments. SCS is
veterinary literature pertaining to bone cysts centers on sev- typically vimentin positive, cytokeratin positive, and CD18 nega-
eral small series of cases or single case reports. 379–381 Affected tive, whereas HS is typically vimentin positive, CD18 positive,
animals are often young and present because of mild or moderate and cytokeratin negative. Thus to avoid confusion, some authors
lameness; however, pathologic fracture can occur through cystic suggest that SCS should be replaced with cytokeratin-positive
areas of long bones leading to severe lameness. There appears to joint associated sarcoma because this better describes the distinc-
be a familial tendency in Doberman Pinschers and Old English tion between HS and the classical SCS. 382 Canine SCS can be
Sheepdogs. These lesions are usually in metaphyseal regions of further subclassified into monophasic or biphasic depending on
long bones, and they can cross an open growth plate; however, the presence or absence of an epithelioid component (biphasic) in
unicameral bone cysts can sometimes be diaphyseal or epiphy- addition to the spindle cells typically noted with SCS. Amputa-
seal. The etiology and pathogenesis are unknown, but it is specu- tion is recommended as more conservative resections result in a
lated that the lesions may be the result of trauma to the growth significantly worse outcome. The overall MST for dogs with SCS
plate interfering with proper endochondral ossification. Others is 455 to 967 days, but STs are also dependent on histologic grade.
have theorized that with the rapid resorption and deposition of The MSTs for dogs with grade I, II, and III SCSs are 365 to 1460
bone occurring in the metaphysis of a young animal, a cyst might days, 156 to 1095 days, and 183 days, respectively. The meta-
develop if resorption is so rapid that a focus of loose fibrous static rate for dogs with SCS is 8% to 32% 383 but up to 91% for
tissue forms. The focus of fibrous tissue may then obstruct the dogs with periarticular HS. Treatment for dogs with periarticular
thin-walled sinusoids causing interstitial fluid to build up and HS should include amputation and adjuvant CCNU as MSTs are
form a cyst. Cysts have been described in bone immediately superior with this multimodality therapy (568 days) than amputa-
below articular cartilage (subchondral bone cysts or juxtacortical tion alone (161 days). 382,384,385
bone cysts). 376,379,381 In these, it is possible to demonstrate a Myxoma sarcomas have a similar radiographic appearance to
direct communication with the articular synovial membrane. SCS but, on histology, typically contain multiple myxomatous
Radiographically, bone cysts are either single or, more com- islands containing widely spaced stellate cells. On gross inspec-
monly, multilocular, sharply defined, centrally located, radio- tion, these tumors are composed of gelatinous nodules that fill
lucent defects in the medullary canal of long bones. Variable the joint cavity and exude viscous fluid when incised. They occur
degrees of thinning of the cortex with symmetric bone “expan- in the stifle and digits most commonly. They are distinguishable
sion” are often a radiographic feature. Diagnosis of bone cyst from SCS and HS because they stain positive for vimentin and
relies on the histologic finding of a thin, fibrous wall lined by flat HSP, variably positive for CD18 (20%–40%), and negative for
to slightly plump layers of mesothelial or endothelial cells. Treat- cytokeratin. Myxomas of the joint are treated with either amputa-
ment consists of meticulous curettage and packing the space tion or local resection (synovectomy) and can have long STs (>2
with autogenous bone graft. years) even with incomplete excision. 384
Aneurysmal bone cysts (ABCs) are spongy, multiloculated
masses filled with free-flowing blood. The walls of an ABC are
rarely lined by epithelium, and the lesion possibly represents an Primary Bone Tumors of Cats
arteriovenous malformation. A proposed pathogenesis of ABCs Incidence and Risk
is that a primary event, such as trauma or a benign bone tumor,
occurs within the bone or periosteum. This event disrupts the Primary tumors involving the bones of cats are rare. An estimate
vasculature, resulting in a rapidly enlarging lesion with anom- of the incidence of all bone tumors in cats is 4.9 per 100,000. 386
alous blood flow that damages the bone mesenchyme. The Between 67% and 90% of bone tumors in cats are histologically
bone reacts by proliferating. As the vascular anomaly becomes malignant. OSAs are the most common primary bone tumor in
stabilized, the reactive bone becomes more consolidated and cats and account for 70% to 80% of all primary malignant bone
matures. The age of affected dogs ranges from 2 to 14 years, cancers of cats. Feline OSA occurs in appendicular and axial skel-
but it has been reported in a 6-month-old dog. 380 Treatment etal sites and extraskeletal sites. OSA occurs in the appendicular
options include en bloc resection and reconstruction or exten- skeleton approximately twice as often as in axial sites; 387,388 how-
sive curettage with packing of the defect with autogenous bone ever, in one study, 55% of 90 skeletal OSA cases were appendicular
graft. and 44% occurred in the axial skeleton. 389 Axial OSA originates